1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus including the zoom lens, and is suitable for use in, for example, a broadcasting TV camera, a video camera, a digital still camera, and a silver-halide film camera.
2. Description of the Related Art
In recent years, in an image taking optical system used in an image pickup apparatus such as a TV camera, a silver-halide film camera, a digital camera, and a video camera, a zoom lens which has a high zoom ratio and high optical performance (high resolution) is demanded.
In order to obtain high optical performance, various aberrations related to image performance at a single wavelength, such as spherical aberration and coma, should be excellently corrected. In order to prevent an image from having color blurring in a case where white illumination light is used, chromatic aberrations should be sufficiently corrected.
In order to expand an image taking region, a zoom lens having a high zoom ratio is required.
In general, when the zoom ratio increases, significant lateral chromatic aberration is generated at a zoom position located on the wide angle side, and significant lateral chromatic aberration and on-axis chromatic aberration are generated at a zoom position located on the telephoto side.
The chromatic aberrations increase as a total lens length (distance between first surface and imaging plane, which is also referred to as total optical length) shortens.
When the chromatic aberrations are to be corrected, it is important to excellently correct not only a primary spectrum but also a secondary spectrum in order to obtain high-quality image performance.
There has been known, as a telephoto type zoom lens, a zoom lens which includes four lens units constituted by a lens unit having a positive refractive power, a lens unit having a negative refractive power, a lens unit having a positive refractive power, and a lens unit having a positive refractive power, which are provided in order from the object side to the image side. There has been known, as the zoom lens including the four lens units, a zoom lens in which chromatic aberrations are corrected using a lens made of an extraordinary dispersion material (see, for example, U.S. Pat. No. 6,414,799, Japanese Patent Application Laid-Open No. H08-248317, U.S. Pat. No. 6,404,561, and U.S. Pat. No. 6,594,087).
A material with large extraordinary dispersion property, such as a fluorite, normally has a low refractive index. Therefore, in order to correct a secondary spectrum using a lens made of such a glass material, it is necessary to suitably set a lens structure and lens material of each of the lens units.
In a zoom lens using a lens made of an optical material such as low-dispersion glass with a large Abbe number, such as a fluorite, when a refractive power of a lens surface is not significantly changed, chromatic aberrations do not change. Therefore, it is important for the zoom lens having the high zoom ratio to employ a lens structure capable of correcting chromatic aberrations and various aberrations such as spherical aberration, coma, and astigmatism over the entire zoom range in a balanced manner.
For example, in order to obtain high optical performance of the zoom lens including the four lens units over the entire zoom range and the entire focusing range (entire object length range), it is important to suitably set a lens structure of the second lens unit for magnification and a lens structure of the first lens unit for focusing. It is also important to suitably set a material of each lens used for the first lens unit and the second lens unit.
However, when the lens structure, for example, power or a material to be used, of the second lens unit or the lens structure of the first lens unit is unsuitably selected, it is difficult to excellently correct the chromatic aberrations to obtain the high optical performance over the entire zoom range and the entire focusing range.